1. Technical Field
This invention relates generally to metal casting and more particularly to controlling heat flow and solidification during casting.
2. Related Art
There are problems in automotive circles today of irregular friction reaction and vibration of rotary friction disc brake rotor and clutch plates that are a direct result from the way in which they are cast. These components are typically cast in vertically split molds with the friction plate oriented vertically and filled through a bottom inlet gate so that the mold cavity is filled from the bottom up. Regardless of where the molten metal enters the vertical mold cavity, gravity causes the low side to fill first and proceeds up and across the casting, developing different thermal gradients as it fills, and thereby setting up different solidification rates of the metal in the cavity, with the first metal introduced eventually rising in the mold cavity to the top and becoming the coolest, and the last metal introduced being at the bottom adjacent the bottom gate and being the hottest of the metal. To make matters worse, there is a riser at the top of the mold. This riser constitutes a mass of molten metal that is the last to solidify, and it sits at the top of the mold adjacent the top of the cavity, and thus creates another hot spot in the center of the cooler upper portion of the. The severe thermal gradient causes the metal to solidify at different rates and yields a variation in the microstructure and stresses around the mass of the casting and friction face(s) of the casting, with the hotter regions being relatively softer than the cooler regions as a direct result of a relatively slower solidification rate of the hotter regions. The relative soft and hard regions in the circumferential direction of a rotary friction plate cause irregular wear, strength and braking resistance over the life of the rotary friction element. The most noticeable result in brake discs is an undesirable pulsing feel of the brakes as the operator of the vehicle slows the vehicle, rather than a smooth, even braking feel.
When friction plates are cast horizontally and gated from the outside perimeter, the problem is less noticeable as the thermal gradients are less pronounced. External inlets and risers around the mold cavity develop similar irregularities but it is the way to get molten metal into more than one region of the mold cavity and gradients tend to blend as they mix across the horizontal surfaces. These thermal gradients during solidification cause uneven structure by areas. Areas of flow through inlets accumulate and contain relatively high temperatures. Areas with heat and high liquid head pressure can move mold walls under these conditions. Areas of riser feed paths are required to maintain relatively high temperatures to compensate for liquid cooling. Areas between the higher temperatures start to solidify well ahead of hotter areas. Irregular temperatures from area to area promote differentials in solidification rates which promote differences in graphite flake size, grain size and hardness as well as variable strengths and stresses which can lead to distortion when relieved with heating and cooling of repeated function or come apart in the soft and relatively weak areas.
Complaints from automotive customers create a necessity and opportunity to find a way to improve the system or eliminate the problem. Any casting that is used as a rotating component is sensitive to variations in microstructure and properties. Several attempts have been made to modify present systems of casting and have improved the situation somewhat but have failed to eliminate the cast structure variation because they still use the vertical (on edge) orientation or widespread inlets and outside inward feeding around horizontal orientation.